"A strong Britain, inside the European Union, remains the best hope in our view for securing Britain's future, creating a more prosperous Europe and protecting a healthy and resilient global economy," they said.

“While Britain will remain an attractive centre for finance even if Britain exits, it should not take for granted its global primacy when it is no longer the gateway to Europe”

"While Britain will remain an attractive centre for finance even if Britain exits, it should not take for granted its global primacy when it is no longer the gateway to Europe," the letter says.

The group of former treasury secretaries - which includes those who have served under both Democratic and Republican presidents - said that while trade deals would eventually be renegotiated, Brexit would "likely disrupt and reduce trade flows at least for a while, curtailing the scale and efficiency benefits from economic co-operation and integration".

Barack Obama is expected to warn against Britain leaving the EU later this month.Credit:
EPA

They added that thanks to the special relationship, during difficult times, they had looked to Britain as a "strong voice and partner" and claimed that "our collective efforts benefited from having a strong Britain within Europe".

The intervention was criticised by Brexit campaigners.

A Vote Leave spokesman told the BBC: "Not content with doing down Britain's economy, No 10 are now soliciting help from across the pond."